Magnetoelastic force measuring means are previously known, for example from Swedish Patent No. 151 267. These force measuring means have a transducer comprising a core with an excitation winding, supplied with alternating current, and a measuring winding. The voltage induced in the measuring winding is usually rectified, suitably with the aid of a phase-sensitive rectifier, and the magnitude of the rectified voltage constitutes a measure of the force applied to the transducer.
A drawback of certain transducers of the above-mentioned kind is, however, that the temperature dependence of the sensitivity and other forms of drift, caused, for example, by amplitude or frequency changes in the magnetizing current, make it difficult or impossible to measure with very high precision.
Swedish Patent No. 430 541 discloses one way of greatly reducing sensitivity drift problems of the above-mentioned kind. In this patent, a reference transducer of the same material and with the same design as the measuring transducer is used. The two transducers are given the same excitation and are arranged in good thermal contact with one another. A spring with a special shape loads the reference transducer with a constant force. The output signal of the reference transducer is rectified in a phase-sensitive rectifier, the output signal of which is compared with a constant reference voltage. The deviation controls the phase position of the control voltage of the rectifier in such a way that the output signal of the rectifier is caused to correspond to the reference voltage. The output signal of the measuring transducer is rectified in a phase-sensitive rectifier, which is controlled by the mentioned control voltage and the output signal of which constitutes the temperature compensated measured signal.
In the device of SE 430 541, thus, the signal flux of the reference transducer, the secondary flux, is measured at a constant applied force from the above-mentioned spring. This flux gives rise to a signal which is kept constant by changing that current at which the phase-sensitive rectifier commutates. This method for reducing the sensitivity drift in magnetoelastic force transducers can reduce the drift problems very drastically. In practice, however, it has proved that this method has certain drawbakcs. A condition for the compensation of the drift according to the method under discussion--with a reference transducer loaded with a constant force--to function satisfactorily is that the force on the reference transducer is prevented from being influenced by the measuring transducer being loaded. This would in such case cause an impermissible variation of the reference signal which is not due, for example, to temperature variations. This has been found to be difficult to achieve in practice; it renders the mounting more expensive and results in a certain amount of rejections.
Because of differences between the measuring transducer and the reference transducer, a pre-trimming is also normally necessary, which may be relatively difficult to achieve.
Otherwise, the specially designed reference transducer and the spring for maintaining a constant force entail a high manufacturing cost.